We aim to determine the three-dimensional structure of N-terminal tetramerization domain of Shaker potassium channel using MAD phasing techniques. From the structure, we aim to understand the structural principle underlying the formation of a functional voltage-gated potassium channel through the tetrameric assembly of subunits. The structure we aim to determine will be the first case, for which an atomic structure is known for any eukaryotic ion channels. Ultimately, the model will provide a ground towards the understanding on the nature of functional heterogeneity of potassium channels in eukaryotic cells, which normally arises not only from multiple genes and splice variants but also from combinatorial mixture of different subunits in forming heterotetramers.